Many types of fuel cells are known in the art, such as solid oxide, molten carbonate, phosphoric acid and proton exchange membrane (PEM) fuel cells. Fuel cells generate electricity by directly converting chemical energy to electrical energy. In a typical fuel cell, an electrolytic medium separates an anode and a cathode. A voltage is produced between the anode and cathode when a fuel is introduced to the anode, an oxidant is introduced to the cathode and the cell is maintained within the correct temperature range. The electrolytic medium allows an ionic species to travel between the cathode and the anode.
The reaction products generated by fuel cells are relatively simple and benign, typically including water and carbon dioxide, thus minimizing environmental concerns. In contrast with fossil fuel based power sources, such as the internal combustion engine, fuel cells are simpler, quieter, non-polluting and have high operating efficiencies. For these and other reasons, fuel cells are considered promising power sources for the future.
In practice, however, operation of a fuel cell stack can be complex. Considerable hardware may be required to support a fuel cell stack, typically comprised of a plurality of individual electrically integrated fuel cell assemblies. Such hardware can include a thermal management subsystem for maintaining the fuel cell stack at the proper temperature, a water management subsystem for handling water generated as a reaction product of operating the fuel cell stack and for maintaining proper humidity throughout the power plant, a fuel subsystem for processing and delivering the fuel reactant to the fuel cell stack, and a blower for delivering the oxidant to the fuel cell stack. Taken as a whole, a fuel cell stack and its operating systems comprise a typical fuel cell power plant.
As understood by one of ordinary skill in the art, the components and subsystems of a fuel cell power plant can vary depending on the application--a phosphoric acid stationary power plant for industrial use will differ from a mobile--(PEM) power plant. Furthermore, a mobile PEM power plant that can be provided with hydrogen as a fuel reactant can differ considerately from a PEM plant for installation in an automobile, which can be required to include a subsystem for producing hydrogen fuel from gasoline. In general, a fuel cell power plant includes those subsystem components necessary for the application for which the power plant is to be used, and that are appropriate to the type of fuel cells incorporated by the fuel cell power plant.
In order to control the temperature within a fuel cell assembly, a coolant is provided to circulate about the fuel cell assembly, usually water. The use of reformed fuels within fuel cell assemblies makes them particularly sensitive to possible water contaminants.
Electrochemical fuel cell assemblies typically employ hydrogen as the fuel and oxygen as an oxidant where, as noted above, the reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the two electrodes formed of porous, electrically conductive sheet material--typically carbon fiber paper. The ion exchange membrane is also known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION.TM., and has a catalyst layer formed thereon which results in a membrane-electrode that promotes the desired electrochemical reaction.
In operation, hydrogen fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the membrane to the cathode and the electrons flow through an external circuit to the cathode. At the cathode, the oxygen-containing gas supply also permeates through the porous electrode material and reacts with the hydrogen ions and the electrons from the anode at the catalyst layer to form the by-product water. Not only does the ion exchange membrane facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers are represented by the equations: EQU Anode reaction: H.sub.2.fwdarw.2H.sup.+ +2e EQU Cathode reaction: 1/2 O.sub.2 +2H.sup.+ +2e.fwdarw.H.sub.2 O
Conventional PEM fuels cells have the ion exchange membrane positioned between two gas-permeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite-polymer composite, or the like. The plates act as a structural support for the two porous, electrically conductive electrodes, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode and cathode, respectively. They are also utilized for carrying away the reactant by-product water during operation of the fuel cell.
When flow channels are formed within these plates for the purposes of feeding either fuel or oxidant to the anode and cathode plates, they are referred to as fluid flow field plates. These plates may also function as water transfer plates in certain fuel cell configurations. When these plates simply overlay channels formed in the anode and cathode porous material, they are referred to as separator plates. Moreover, the plates may have formed therein reactant feed manifolds which are utilized for supplying fuel to the anode flow channels or, alternatively, oxidant to the cathode flow channels. They also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated as a by-product, from the fuel cell. Alternatively, the manifolds may be external to the fuel cell itself, as shown in commonly owned U.S. Pat. No. 3,994,748 issued to Kunz et al. and incorporated herein by reference in its entirety.
The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. Multiple electrically connected fuel cells consisting of two or more anode plate/membrane/cathode plate combinations are referred to as a fuel cell stack. A fuel cell stack is typically electrically connected in series.
Recent efforts at producing the fuel for fuel cell assemblies have focused on utilizing hydrogen rich streams produced from the chemical conversion of hydrocarbon fuels, such as methane, natural gas, gasoline, methanol or the like, into hydrogen. This process requires that the hydrogen produced must be efficiently converted to be as pure as possible, thereby ensuring that a minimal amount of carbon monoxide and other undesirable chemical byproducts are produced. This conversion of hydrocarbons is generally accomplished through the use of a steam reformer or an auto-thermal reformer. Reformed hydrocarbon fuels frequently contain quantities of ammonia, NH.sub.3, as well as significant quantities of carbon dioxide, CO.sub.2. These gases tend to dissolve and dissociate into the water which is provided to, and created within, the fuel cell assembly. The resultant contaminated water supply may cause the conductivity of the water to increase to a point where shunt current corrosion occurs in the coolant channels and manifold leading to degradation of fuel cell materials, as well as reducing the conductivity of the PEM and thereby reducing the efficiency of the fuel cell assembly as a whole.
As disclosed above, the anode and cathode plates provide coolant channels for the circulation of a water coolant, as well as for the wicking and carrying away of excessive water produced as a by-product of the fuel cell assembly operation. The water so collected and circulated through a fuel cell assembly in the coolant channels is susceptible to contamination and may therefore damage and impair the operation of the fuel cell assembly as the contaminated water circulates throughout the fuel cell assembly.
It is therefore necessary to provide a system which may protect the fuel cell assembly from water contamination, such as is described in commonly owned U.S. Pat. No. 4,344,850 issued to Grasso and incorporated herein by reference in its entirety. Grasso's system for treating the coolant supply of a fuel cell assembly, as illustrated in FIG. 1 of U.S. Pat. No. 4,344,850, utilizes a filter and demineralizer for purifying a portion of the coolant supplied to the fuel cell assembly. A deaerator is also utilized to process the condensed water obtained from a humidified cathode exit stream. As discussed in Grasso, the heat exchange occurring between the coolant stream and the body of the fuel cell assembly is accomplished according to commonly assigned U.S. Pat. No. 4,233,369, issued to Breault et al., incorporated herein by reference in its entirety. It is important to note that Grasso's coolant system does not provide for the cleansing of the coolant stream as a whole. This is due to the fact that the coolant conduits in Grasso, being fashioned from copper or the like, are not in diffusable communication with the body of the fuel cell assembly and as such, the coolant stream does not receive contamination from, inter alia, the CO.sub.2 or NH.sub.3 when present in the reformed fuel stream. The burden of cleansing the coolant stream in Grasso is therefore born solely by the filter and demineralizer and would result in greater wear on these components and hence greater repairs and more frequent replacements, if diffusable contamination occurs.
Degassifiers have also been utilized in fuel cell power plants to assist in the removal of the CO.sub.2 or NH.sub.3 gasses from the coolant supply, such as illustrated in commonly assigned co-pending U.S. application Ser. No. 09/295,732, filed on Apr. 20, 1999, entitled "WATER TREATMENT SYSTEM FOR A FUEL CELL ASSEMBLY", herein incorporated by reference in its entirety.
The inclusion, however, of separate deaerators and degassifiers in a fuel cell power plant increases the complexity of such a system, as well as corresponding increase in the energy required to operate such a system.
With the forgoing problems and concerns in mind, it is the general object of the present invention to provide a fuel cell power plant with a method and apparatus for removing contaminants from the coolant supply which overcomes the above-described drawbacks.